Insulated-gate thin film transistors are well known, see, e.g., Sze, Physics of Semiconductor Devices, pp. 567-86 (1969). Thin film transistors utilize evaporated semiconductor, metal and insulator layers to form a device which functions essentially as an MOS field-effect transistor. Unlike a field-effect transistor, a thin film transistor has, however, source and drain electrodes essentially ohmically connected to the semiconductor channel, and may have a semiconductor channel of amorphous or polycrystalline material.
Insulated gate transistors have two operating modes: the depletion mode and the enhancement mode. In the depletion mode, the conducting channel exists with zero gate-bias, and the insulated gate is reverse biased to deplete the charge carriers of the channel and reduce the channel's conductance. In the enhancement mode, the insulating gate is forward biased which enhances the charge carriers of the channel and increases the channel's conductance.
Even in the enhancement mode, thin film transistors have a certain drain current at zero gate voltage. This leakage current (I.sub.OD) constitutes a part of the total drain current (I.sub.D TOTAL) which cannot be modulated with gate voltage (V.sub.G). To a first approximation, the drain leakage current is a constant independent of the applied gate voltage. The total drain current is, therefore, given as I.sub.D TOTAL = I.sub.D + I.sub.OD where I.sub.D is the portion of the drain current which can be modulated. For a properly operational insulated-gate thin film transistor, it is thus necessary that the leakage current (I.sub.OD) be much less than the total drain current (I.sub.D TOTAL).
Generally, the leakage current has been minimized by minimizing the thickness of the semiconductor layer. However, there is a minimum thickness of semiconductor material which can be deposited reproducibly. Thus, the presence of leakage current continues to be a restricting factor on insulated-gate thin film transistors. Thin film transistors built with tellurium semiconductor material having high carrier densities and mobilities have appreciable leakage currents even at minimum layer thickness.
The present invention overcomes the difficulty of prior thin film transistors. It provides an insulated-gate thin film transistor with substantially reduced drain leakage current and in turn provides a thin film transistor utilizable in applications where thin film transistors could not previously be used.